In TDD CDMA based wireless communication systems, there are mainly two intra-cell interferences: one is MAI (multiple access interference), caused by sharing of the same frequency band by different users and the loss of orthogonality between the spreading codes allocated for different users due to the multipath channel effects; another is ISI (inter-symbol interference) between different paths of the same user, caused by multipath propagation.
To effectively mitigate MAI and ISI, JD (joint detection) is introduced to conventional TDD CDMA communication systems. JD takes full advantage of the spreading codes, channel fading, signal delay and other information about the user signal, so it can improve signal transmission quality in the cell and increase TDD wireless communication system capacity. Furthermore, JD is suitable for existing HCR (HCR: High Chip Rate, 3.84 Mchip/s) and LCR (LCR: Low Chip Rate, 1.28 Mchip/s) TDD systems, and even higher chip rate candidate proposal of 7.68 Mchip/s discussed now by 3GPP. Thus, it can be seen that, JD technique has become one of the key technologies in current TDD CDMA systems.
T3G, a JV organized by Datang, Philips and Sumsang to develop TD-SCDMA handset solution designs, has adopted JD algorithms of ZF-BLE (ZF-BLE: zero forcing block linear equalizer) and MMSE-BLE (minimum mean square error block linear equalizer) in her first 3G mobile products.
However, the implementation of ZF-BLE and MMSE-BLE algorithms needs to know as precondition the spreading codes of all active UEs. For the base station, this won't pose as a problem, because the base station is responsible for resource allocation and thus can know the spreading codes of all users very easily. But for a UE, it only knows its own spreading code and has no knowledge of the spreading codes of other UEs sharing the same timeslot. Thus it is no easy job to implement JD algorithms for UEs.
To implement JD algorithms in UEs, one solution is to add an additional “active-code detection” module in the receiver of a TD-SCDMA handset so that information about other UEs' spreading codes can be recovered in a single UE, which can be referred to “Performance of active codes detection algorithms for the downlink of TD-SCDMA system,” IEEE Inter. symposium on circuit and systems (ISCS), Vol. 1, 2002, pp. 613-616, by Kang Shao-li et al, and “Technology requirements of the 3GPP-TDD terminal,” IEEE 2000 Inter. conf. on 3G Mobile communication technologies, pp. 89-93 by S. Kourist et al. Unfortunately, this solution using “active-code detection” module has very poor performance in some cases, especially in the environment of lower vehicle speed and multipath fading, which causes severe system capacity loss.
There is also another optional solution of adopting equalized single user detection JD algorithm called MMSE-BLE-SD, which can be referred to “Data detection algorithms specially designed for the downlink of CDMA mobile radio systems,” IEEE International Conf. on Vehicle Technology (VTC), Vol. 1, May 1997, pp. 203-207, by A. Klein. Compared with ZF-BLE/MMSE-BLE, the performance of MMSE-BLE-SD is a little poorer, but its advantage is only need to know the spreading code of the UE. However, MMSE-BLE-SD algorithm also has to know the ACN (active code number) allocated in the same timeslot as the UE in advance. Although the ACN can be estimated at the UE by some special algorithms, the single-user receiver's complexity and power consumption will be increased heavily due to the added huge computation loads.
In fact, the above two problems can both be easily overcome through sending the mandatory information of the spreading codes or the ACN via some downlink channels to each UE by the base station.
A method of transmitting the related spreading codes information from the base station via common control channel (such as BCH) to UEs, is described in the patent application document entitled “Mobile station enabled for use of an advanced detection algorithm”, submitted on Jan. 13, 2003, filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. and EPO Application Serial No. 03075075.6. According to the method as disclosed in this patent application, the spreading code associated with a midamble can be obtained from the midamble allocation information. However, it is restricted to the case of knowing the association relationship between midambles and channelization codes, that is, the so-called “default midamble” case. There are two other midamble allocation cases in the 3GPP TDD standard: (i) the “common midamble”, wherein all users sharing the same timeslot use the same midamble; (ii) the “midamble allocation by signaling from higher layers”, wherein there is no fixed relationship between the allocated spreading codes and midambles, which can be referred to 3GPP Technical Specifications 25.221, “Physical Channels and mapping of transport channels onto physical channels (TDD)”, (Release 4), March, 2001. In these two cases, the method as disclosed in the patent application has some restricts.
A method to broadcast the CAI (codes allocation information) directly on downlink common control channel (such as BCH) is proposed in the patent application document entitled “Method and apparatus for supporting P2P communication in TDD CDMA system”, filed by KONINKLIJKE PHILIPS ELECTRONICS. N.V. and the Application Serial No. 03110415.0. According to the method as disclosed in this patent application, common control channel has fixed position in a radio frame or sub-frame (for instance, BCH is in TS0), thus every UE can receive the CAI and perform JD by using the CAI. But, a problem will arise when BCH is used to transfer the information. The repetition period of BCH is at least 80 ms (8 radio frames) or even longer (160, 320, or 640 ms, to be decided by the higher layer).
When CAI varies rapidly, it's likely too late to update the information. Moreover, if a large mount of CAI has to be transmitted over BCH every repetition period, continuous overloads on BCH will happen inevitably.
In fact, the change of CAI only occurs in three situations: first, early when communication connection is being established, the base station allocates spreading codes to new UEs; second, during communication course, change of users in the same timeslot occurs, for example, other users enter or leave the timeslot and thus the allocation of spreading codes changes accordingly; third, communicating UEs handover to other cells and release the spreading codes in the former cell. It can be seen from the three cases that, the change of CAI only occurs in certain time period. If the system is very stable, it is of no necessity to transmit the CAI every repetition period over BCH. Moreover, the change of CAI only affects UEs associated with the same timeslot, but has no impact on any other UEs working in other timeslots.
Therefore, a more effective method is needed to provide CAI so that the UEs can perform JD algorithms by using the CAI.